In Optical Character Recognition (OCR) technology in which image information of an original such as a paper document is read by an image reader such as a scanner to create electronic data, an automatic document feeder is frequently used. In the automatic document feeder, each original is automatically extracted and separated from a bundle of originals sequentially one by one and is conveyed or fed to the image reader so that the image information of the original image is read by the image reader. The structure in which this type of automatic document feeder is combined with the image forming apparatus is well known.
Typically, the automatic document feeder includes a pair of rollers to convey the original to the image reader upstream from the original reading line position of the image reader and another pair of rollers to discharge the original conveyed to the image reader therefrom.
In the automatic document feeder including such roller pairs, when the original is read to obtain the image information, upon the original entering between the rollers and going out of the rollers, the original receives conveyance load and the original conveyance speed fluctuates. The change in the original conveyance speed occurs similarly at an original reading line position, which causes the image information to be shifted and the image to be output on the recording medium or sheet with color shifted.
Specifically, color shift occurs in the image reader or device in which the RGB image reading line (scan line) is not disposed at the same place relative to the image conveyance direction and when fluctuation in conveyance speed occurs on the RGB image reading line. In addition, it is understood that such irregularity in speed occurs due to deformation of the conveyance materials such as the original or the conveyance roller pair when a drastic load change occurs with respect to the conveyed original. Typical times at which such drastic load changes occur are when a leading edge of the original enters between the conveyance roller pair closest to and downstream from the image reading line, and when the original exits the conveyance roller pair closest to and upstream of the image reading line.
Conventionally, various measures have been thought out to improve this type of color shift. For example, by increasing the diameter of the rollers used as the roller pair, the shock given to the roller pair when the original enters between rollers and therefrom is minimized. However, when the roller diameter is increased, the automatic document feeder cannot be downsized and as a result, the image forming itself cannot be downsized contrary to the requirement of the users.
Another approach is to reduce an interval between the conveyance paths immediately before the original enters into the image reader and immediately after the original exits the image reader. However, designing of the conveyance interval within such a range so as not to clog the original considering the thermal deformation and assembly errors is difficult, and further, a method to eliminate the conveyance path interval by using an elastic member such as mylar (registered trademark) has a problem in that the Mylar™ itself becomes a load when the original enters between rollers.
Yet another approach involves forming the original conveyance path maximally linearly and disposing the conveyance roller pair which is near the image reading line in the vicinity of the reading line. The above measure has a disadvantage in that, due to its importance to the structure of the automatic document feeder, the application of such a design is not allowed. Another problem is that the roller pair comes to bite the reading device. Another approach is to replace the reading white board mounted on the reading line with a belt or a roller, but doing so makes the device complicated and expensive.
JP-S64-17749-A discloses an image reading apparatus including an original discharge roller having an expanded diameter part and a driven roller driven to rotate accompanied by the original discharge roller. The expanded diameter part has a diameter larger than that of the original discharge roller, is less rigid, and is disposed at least at one side of the outer periphery of an outer diameter of the original discharge roller, the outer diameter defining a predetermined reading speed. The invention disclosed by JP-S64-17749-A aims to improve stacking of sheet originals by installing a guide member for leading the original to the contact position between the original discharge roller and the driven roller and the outside diameter in the expanded diameter part, in the original discharge part. However, the disclosed invention may not prevent fluctuations in the original conveyance speed when the original strikes between the roller pair and exits therefrom.